This invention relates to systems for receiving voice and data signals and, more particularly, to the detection of composite signals in such systems.
The present application is related to copending application Ser. No. 08/231,566 filed on the same date as this application, entitled, "Method And Apparatus For Composite Signal Separation And FM Demodulation", by inventor Duane L. Abbey, attorney docket number 92CR037/KE, incorporated herein by reference.
A variety of apparatus and techniques currently exist for processing FM signals in communication systems. In those situations in which the signal-of-interest has co-channel interference or more than one co-channel signal is desired, prior art solutions to such instances generally require duplicative equipment and increased processing time.
A typical receiver is comprised of one or more phase-locked loops (PLL) that may include fixed signal limits. The PLL may take the form of an analog or digital implementation, depending upon design choice and available resources. Generally, a PLL is dedicated to suppress noise by locking to the signal. The PLL contains a voltage-controlled oscillator, whose output could be multiplied with the input signal and integrated to establish a relative measure of noise level in the input signal. Additional PLLs may be included that are bandwidth tailored for the specific demodulation scheme desired. Some such systems may include a threshold (or thresholds) above which the detected noise level will cause the output to be squelched.
Generally, phase modulation is limited to data modulation signals (PSK) because coherent carrier recovery is next to impossible for non-predictable phase reference signals. The two main types of PSK demodulators used are coherent (carrier tracking) demodulators and non-coherent (differential phase change) demodulators. Of the two types, coherent demodulation provides superior noise performance as long as carrier phase synchronism is maintained.
One type of coherent PSK demodulator uses a signal squaring circuit (BPSK) or signal squaring circuit pair in cascade (QPSK) to recover a continuous carrier and a PLL to reduce the frequency back to the transmit frequency. The recovered carrier can be multiplied by the input signal to recover the transmitted data. This approach does have an initial phase ambiguity problem that can require multiple demodulators and recovered carrier offsets to resolve.
A more common type of coherent PSK demodulator uses a demodulated data remodulator PLL. This type of demodulator in its most common form is called a Costas loop. Basically the demodulated data is remodulated with the modulated data signal to recover the carrier. If the recovered carrier is in synchronism, the data will have been demodulated correctly (assuming good S/N). The remodulation phase locked loop seeks to maintain recovered data integrity and thereby maintain carrier synchronism. This approach also has an initial phase ambiguity problem, e.g. a QPSK loop can have the I channel output locked on to any of the following, I modulation signal, -I modulation signal, Q modulation signal, and the -Q modulation signal.
Co-channel noise and interference of sufficient amplitude can quickly break carrier phase synchronism and cause loss of data in either coherent demodulator approach. If the transmitted data was not differentially encoded, the whole data transmission could be lost from a single momentary loss of phase synchronism.
Accordingly, there exists a need for an improved composite signal separator and demodulator capable of handling a wide array of received signals. The demodulator needs to be able to reliably recover PSK data without loss of carrier phase synchronism.